A. The Field of the Invention
This invention relates to prosthetic structures and corresponding surgical methods used to relieve pain caused by disorders of the hip joint. More particularly, the invention relates to prostheses and methods for the subtotal dome arthroplasty of the hip joint.
B. Background Discussion
The hip is one of the most versatile joints of the human body and serves an essential function in allowing an individual to lead a normal life. The human hip joint performs its function much better than any device heretofore designed by human engineers. The hip joint withstands forces which are not readily apparent to those unfamiliar with orthopedics. For example, during ordinary walking, the hip joint is routinely subjected to dynamic forces nearly four times greater than body weight. The dynamic forces on the hip joint may be as great as ten times body weight during activities such as running or jumping.
The bones of the hip joint, when functioning properly, move together with very little friction. To function properly, a healthy hip joint requires an intact layer of hyaline cartilage, the material which makes up the articular cartilage on the opposing surfaces of the joint. Also, the bones of the joint must be in proper alignment and the synovial membranes must produce suitable amounts of lubricating (synovial) fluid. Furthermore, the joint structures must prevent the bones from being placed in an abnormal position.
FIG. 1 is an anterior (that is, taken from the front of the body) cross-sectional view of the human hip. In FIG. 1, the semicircular shape of actabulum 11 can be seen. The upper leg bone, or femur 16, which can be seen just below illium 10, is the longest and strongest bone in the body. The upper end of the femur is provided with a head 14, a neck 18, a greater trochanter 17, and a laser trochanter 15. The speroidally-shaped femoral head is shown in FIG. 1 in a "normal" relationship with the acetabulum. It should be appreciated that FIG. 1 is not intended to show the exact structures of the hip joint, since this structure varies somewhat from individual to individual, but to show the relationship of the major components which make up the hip.
FIG. 1 also illustrates the acetabulum articular cartilage 12 and femoral head articular cartilage 19. A space is shown extending between the entirety of the two articular surfaces. This "joint space" may or may not be present in a particular hip depending upon the condition of the hip. Normally, the articular cartilage is smooth and intact. When the articular cartilage is damaged, pain and an accompanying restriction of motion will usually result.
The femur is provided with a femoral neck which may be up to five centimeters long. The femoral neck separates the shaft of the femur from the femoral head. This arrangement allows the femur a substantial degree of movement without interference from the bones making up the pelvis.
The greater trochanter serves as an attachment point for various muscles and ligaments. The lesser trochanter serves a similar purpose. Round ligament 13 is thought to provide a passage for the blood vessels to the femoral head and also assist with spreading of synovial fluid over the joint surfaces to lubricate and nourish the cartilage. The versatility of the hip joint can be appreciated by realizing that the normal motion of the hip joint includes flexion and extension (rotation in forward and rearward directions, respectively) and adduction and abduction (motion towards the median (center) line of the body and away from the center line of the body, respectively).
While the hip joint generally serves its purpose very well, various disorders of the hip cause a great deal of pain and loss of mobility and function to those who are afflicted with such disorders. Some hip disorders are congenital; that is, they are present at birth. Other disorders of the hip are brought on by bacterial infections which may occur at any age. Perhaps the most widespread disorder of the hip is arthritis. The term "arthritis" is generally used as a common name for the effects of several degenerative hip disorders.
Of the various types of arthritis, osteoarthritis is perhaps the most common. Osteoarthritis is a degenerative "wear and tear" process that affects substantial numbers of people. The final result of unchecked osteoarthritis is damaged articular cartilage which in many cases causes extreme pain as the damaged cartilage surfaces are rubbed together during joint movement. It has been estimated that between 8% and 15% of the populations in developed countries, with higher percentages prevalent in older populations, suffer from some degree of osteoarthritis.
One disorder of the hip which appears to lead to osteoarthritis is known as "congruence". Congruence is a term used to describe a condition in which the shape of the femoral head and the shape of the acetabular socket become matched so that dome area of the acetabulum and the femoral head are nearly always in contact. Congruence of the hip can cause increased wear on the joint surfaces. Several of these disorders and other conditions are explained in more detail hereinbelow.
Osteoarthritis may also involve the development of abnormal bony outgrowths on the joint surfaces known as osteophytes. An osteophyte consists of a lump of "cancellous" tissue (tissue having a lattice structure similar to the spongy tissue of the bone) which is capped by a sheet of soft tissue. Commonly, cysts also form on the femoral head and in the acetabulum of the hip joint. These cysts often form just under the articular cartilage and result in a great deal of pain.
Generally, osteoarthritis affects people past the age of 60 years without providing an easily recognizable single cause. However, osteoarthritis may develop in younger people due to a congenital condition or disease. Furthermore, traumatic injury may cause the development of an osteoarthritic condition.
Another hip disorder is osteonecrosis, or death of a portion of a bone, which is due to an insufficient blood supply to part of, or the whole of, a bone. Osteonecrosis may be brought on by excessive alcohol consumption, administration of particular drugs, old age, or as a result of osteoarthritis.
In the prior art, several methods have been used for alleviating the pain and improving the function of a hip joint afflicted with a degenerative disorder such as osteoarthritis.
Perhaps the earliest surgical procedure used to reduce pain due to a disorder of the hip joint was "ankylosing," or fusing, the joint. This alternative, generally called arthrodesis, alleviates pain in a diseased hip joint but also prohibits any proper functioning of the joint. Thus, arthrodesis is generally not an acceptable procedure of relieving hip pain to most patients. In fact, hip surgery is quite often carried out in order to remedy a hip which has become ankylosed, stiff, or immovable.
In some other cases, "debridement" of a hip joint may be helpful. Debridement of the joint usually consists of removing unwanted bony spurs and loose pieces of bone and cartilage within the joint cavity. While this procedure is helpful in some cases, the most common cause of pain and loss of function is due to degeneration of the hip joint rather than abnormal growths or debris in the joint.
Osteotomy, which generally refers to the cutting and resetting of a bone, has also been used in an attempt to alleviate pain and restore function of the hip joint. By cutting and resetting the femur, for example, it may be possible to reorient the head of the femur within the acetabulum such that portions of the femur head not affected by the degenerative disorder are used as weight-bearing surfaces. However, in the case of osteoarthritis, the surfaces of both the acetabulum and the head of the femur are generally involved in the degenerative condition. If the surface of the acetabulum has been damaged, repositioning of the femoral head will probably not provide relief.
Because of limitations of the foregoing procedures, one of the most common procedures used in treatment of hip disorders is the implantation of an artificial joint component. This procedure is known as "arthroplasty." Arthroplasty has been one of the major areas of advancement in hip surgery during the past quarter century. Hip arthroplasty has included techniques known as interpositional arthroplasty, partial arthroplasty, and total arthroplasty.
Interpositional arthroplasty of the hip joint generally involves interposing a layer of material between the two opposing articular surfaces of the joint. For example, materials such as muscle, fibrous tissue, celluloid, silver plates, rubber sheets, magnesium, zinc, decalcified bones, and pig's bladder have all been used in interpositional arthroplasty of various joints. Cup-shaped structures made from gold foil, glass, or VITALLIUM.RTM. (a cobalt-chromium alloy) have also been interposed between the head of the femur and the acetabulum. Even further attempts have been made to encase the femoral head within a metallic shell and also line the acetabulum with a cup comprised of a plastic-like material.
Partial arthroplasty involves the replacement of one of the two opposing articular joint surfaces. For example, this procedure is used where the femoral head has been damaged but yet the acetabulum is otherwise normal. In such a case, it may be beneficial to replace the femoral head with an artificial prosthesis which will work in conjunction with the natural acetabulum. Partial arthroplasty has met with only limited success.
The most common arthroplasty procedure used to alleviate pain and restore hip function is total hip arthroplasty, also called a total hip replacement. While many different styles of hip replacement prosthesis have been implanted in patients, they generally resemble the prosthesis illustrated in FIG. 2. FIG. 2 also illustrates the femur and a portion of the pelvis in cross section in order to best show how the components of a total hip replacement are implanted in the body.
Conventional total hip replacement involves a complete internal amputation of the hip joint as suggested in FIG. 2. The conventional surgical procedure used during total hip replacement involves making a surgical incision to provide an approach to the hip. Once the hip is exposed, the joint is dislocated so that the femoral head and acetabular socket can be accessed. The femoral head and neck are then amputated. Often, the greater trochanter 32 is removed and reattached at a lower point by the use of wires 34. Once the femoral head and neck have been removed, the femoral canal (the central core of the bone, generally indicated at 26) is reamed so as to provide a cavity into which stem 36 of femoral component 20 may be inserted. The femoral canal is reamed so that its diameter is significantly larger than the diameter of femoral component stem 36.
The most commonly accepted method of fixing the femoral component to the femur is by polymethyl methacrylate (PMMA). PMMA is a two-component acrylic cement which has the advantage of exhibiting a rapid setting time. After mixing the two components, the femoral canal is "packed" with unset PMMA. The stem 36 of femoral component 20 is then inserted into the femoral cavity and the femoral component 20 is held in the proper position until the PMMA has set. Since the femoral canal has been reamed out to a larger diameter than the shaft of the femoral component, the PMMA cement serves as a "grout" 36, interfacing between the shaft and the remaining bone.
The femoral components are available in a variety of sizes and styles, but nearly all those used presently include stem 22, neck 28, and ball-shaped head 30 portions similar to those pictured in FIG. 2. Most of the prosthetic femoral components used presently are fabricated from a cobalt-chromium steel alloy or a titanium alloy.
Ideally, a prosthetic femoral component should exhibit characteristics identical to that of living bone, although conventional prostheses have not equalled normal bone tissue. Problems of fatigue, breakage, and loss of fixation of the femoral component are common following total hip replacement.
Implantation of the acetabular component also requires significant alteration of the bone structure. The acetabulum is first reamed out to provide a cup-shaped cavity into which the acetabular component, generally indicated at 22 in FIG. 2, will be fixed. Conventional acetabular components 22 used in total hip replacements are relatively large. Presently, most acetabular cups are fabricated of ultra-high molecular weight polyethylene (UHMWP). The acetabular component 22 is fixed within the reamed out cavity by PMMA adhesive once again acting as a grout.
After femoral component 20 and acetabular cup 22 have been implanted, the greater trochanter 32, if previously removed, is reattached using wires 34 at a point lower on the femur so as to provide a mechanical advantage more favorable to the total hip prosthesis. The joint is then reduced and the surgical incision closed.
While the conventional total hip prosthesis procedure has been popularly accepted, there still exist major risks and drawbacks that accompany its use. First, the total hip replacement procedure can be characterized as an internal amputation of the femoral neck, head, and acetabulum. A great deal of bone, which in many cases is healthy and potentially usable, is removed and lost. Furthermore, to insert the femoral component, the femoral canal must be severely invaded. The femoral canal is a region with substantial blood flow and numerous blood vessels; invading this area causes a great deal of blood loss. It is not uncommon for a patient to require four to eight units of blood during the procedure. A total hip replacement, even when efficiently carried out, is a complicated procedure often requiring the patient to remain in surgery for a prolonged period of time.
As mentioned earlier, the most common method of fixing the components of a total hip prostheses in place is by the use of polymethyl methacrylate (PMMA). PMMA cement is prepared by mixing two components together which harden into a solid mass by way of a chemical process. One of the two components is a fine granular powder of prepolymerized polymethyl methacrylate and the other component is a liquid monomer.
One of the constituents of the liquid monomer is N,N-Dimethyl-Para-Toluidine (DMPT), a toxic material. Other monomer ingredients also exhibit adverse effects on humans. Thus, the introduction of the mixed, but as yet unset, PMMA cement mixture into the femoral canal, an area rich in blood vessels, presents the potential of introducing a significant amount of toxic materials into the blood stream Known reactions to PMMA cement include hypotension and even circulatory system collapse.
Aside from the immediate hazards that attend the use of PMMA cement, concern has also been expressed that there may be long term toxicity, hypersensitivity, and carcinogenicity resulting from the materials that make up the prior art total hip prosthesis, including cobalt, chrome, titanium, and polyethylene. In view of the uncertainty of the effects of long term use of these materials within the human body, it has been considered advisable to reduce the contact between these materials and the body as much as possible.
In any surgical procedure there is the potential that infection may occur due to entry of microorganisms into the surgical wound. Devastating infections are particularly difficult to prevent in total hip replacement procedures due to the extensive invasion of the body that is required. Special surgical techniques have been developed which reduce the risk of an infection to the patient. Unfortunately, these surgical techniques require far greater care than other types of surgical procedures, and in some cases are extremely cumbersome.
As mentioned previously, the stresses on the hip joint during ordinary activities are very high. During strenuous activities those stresses are increased several fold. These high stresses result in several mechanical difficulties in a patient fitted with a conventional total hip prosthesis. For example, it is not uncommon for the femoral component to become dislocated from the acetabular component. Alternatively, the components may fail, i.e., fracture or break, due to the stresses placed upon them. A common difficulty is loosening of the components from the surrounding bone. Generally, problems such as loosening or failure are particularly acute with the femoral component. Furthermore, not only does loosening or failure of the prosthesis present a danger to the patient, but due to the procedures performed on the surrounding bone, and the additional stress placed on the bone because of the use of the prosthesis, those bone structures supporting the prosthesis components may be subject to an increased chance of fracture and undue wear.
Because of these difficulties the maximum useful life of an implanted prosthesis is frequently less than 8 to 12 years. In individuals less than 40 years of age, the average useful life of a conventional prosthesis is often only about 4 to 6 years.
Once total hip replacement has occurred, revision of the procedure (revision being the term used to describe when the prosthesis is replaced), can be extremely difficult due to the amount of bone mass which was removed during the original procedure. Also, a revised total hip replacement has a much shorter useful life than the original replacement. Still further, the bone structure, as mentioned earlier, may become weakened due to the additional stresses placed upon them by the prosthesis and due to any effects which at the present time are unknown, such as delayed hypersensitivity to the materials used in the prosthesis Because of the relatively short expected life of a prosthesis multiple replacements would be necessary over the lifetime of a younger patient.
In view of the above difficulties inherent in the total hip replacement structures and procedures as carried out in the prior art, it would be a significant advance in the art to provide prosthetic structures and methods to reduce or eliminate pain inherent in an osteoarthritic hip joint, and to improve the function of such a hip joint, while avoiding the hazards and risks inherent in the presently used total hip replacement procedure.
It would be a further advancement in the art to provide prosthetic structures and a simplified surgical procedure which are as noninvasive as possible and minimize the removal of healthy bone mass. It would be particularly beneficial if blood loss during hip replacement could be substantially decreased.
Furthermore, it would be a significant benefit to provide hip prosthetic structures and accompanying procedures which minimize the amount of nonbiological material to be implanted in the body.
Still further, providing hip prosthetic structures and procedures which present a minimal risk of loosening or failing over a long period of time and subject to the problem of dislocation presently observed in the conventional prosthetic devices, would be a welcome advance in the art.
It would also be a significant advance in the art to provide hip prosthetic structures and procedures which would allow the choice of several alternatives if revision of the initial implant becomes necessary.
Furthermore, providing hip prosthetic structures which are simpler to manufacture and cost less than those prostheses available in the prior art, would be another advance in the art.
These and other benefits are obtained by the structures and methods for subtotal hip dome arthroplasty of the present invention.